1. Field of the Invention
The present invention relates to a liquid-filled vibration damping device for use in automotive vehicles and industrial machinery.
2. Description of the Related Art
Liquid-filled vibration damping devices are advantageously used for engine mounts, transmission mounts, suspension bushes, motor mounts and the like in automotive vehicles and industrial machinery.
Referring to FIG. 4, for example, an engine mount (see, for example, JP-A-2005-337348) includes an upper retention member 1 to be attached to an engine (vibration source), and a lower retention member 2 to be attached to a vehicle body or a frame. The upper retention member 1 and the lower retention member 2 are unified together by a molded vulcanized rubber member 3 and a tubular member 4. A diaphragm 5 is attached to the lower retention member 2, and a liquid-filled space in which liquid P is sealingly contained is defined between the diaphragm 5 and the molded vulcanized rubber member 3. The liquid-filled space is partitioned into two vertically arranged chambers, i.e., a primary liquid chamber 9 and a secondary liquid chamber 10, by a partition member 8 formed with an orifice 7. The liquid P flows between the two chambers 9, 10 through the orifice 7. In FIG. 4, reference numerals 11 and 12 denote mounting bolts.
With this arrangement, even if the molded vulcanized rubber member 3 receives vibrations of the engine, vibrations to be transmitted to the vehicle body are damped by a vibration absorbing effect provided by the resilience of the molded vulcanized rubber member 3 and a fluid effect of the liquid P in the liquid-filled space.
Further, a fluid-filled vibration damping device (see, for example, JP-A-2004-169750) is proposed, which has a region filled with a non-compressive fluid and is improved in vibration damping capability by adding a predetermined volume of air into the fluid-filled region. In order to facilitate the addition of the air by means of simplified equipment and to accurately control the volume of the air to be added, the fluid-filled vibration damping device has a gas retaining hole which opens in the fluid-filled region to retain the gas. In assembling the fluid-filled vibration damping device, a plurality of combinational members cooperatively defining the fluid-filled region are combined together in the non-compressive fluid with the gas retained in the gas retaining hole, whereby the fluid-filled region is formed as including the gas retaining hole in which the gas is retained.
With recent drastic technological innovation in the automotive industry, very quiet driving as compared with the prior art has been achieved. Problematically, the engine mount is liable to suffer from abnormal noises and vibrations when great vibrations are applied to the engine due to undulations on a road. This is because the internal pressure of the pressure receiving chamber (primary liquid chamber) of the engine mount is suddenly drastically reduced to form cavitation bubbles in the pressure receiving chamber when the elastic rubber member of the engine mount is significantly resiliently deformed due to the vibrations applied to the engine and, when the bubbles disappear, impulse waves are generated and transmitted to the vehicle body to cause the abnormal noises and the vibrations.
For suppression of the cavitation, there have been proposed a method such that a pressure difference between the primary liquid chamber 9 and the secondary liquid chamber 10 is reduced by providing a relief valve or a like device, and a method such that reduction in the internal pressure of the primary liquid chamber 9 is suppressed by adding liquid having a higher vapor pressure (or a lower boiling point) to the fill-in liquid P and vaporizing the higher vapor pressure liquid at a negative pressure (see, for example, WO2009/154222).
With the provision of the relief valve or the like device, however, the pressure difference between the primary liquid chamber 9 and the secondary liquid chamber 10 is reduced when the valve is opened for the suppression of the cavitation. Therefore, the amount of the liquid P flowing through the orifice 7 for the damping is reduced, thereby reducing the vibration damping capability. This makes it impossible to sufficiently provide the intended vibration damping effect.
In the method in which the higher vapor pressure liquid is added as disclosed in WO2009/154222, the liquid to be added has a lower boiling point. Where the engine mount is used in an environment such as an engine room likely to be heated to a higher temperature, therefore, the added liquid boils even at a non-negative pressure, and a large volume of gas is constantly present in the liquid chambers. This reduces the vibration damping capability, and causes expansion and rupture of the diaphragm, resulting in a problem associated with the function of the product.
In view of the foregoing, it is an object of the present invention to provide an excellent liquid-filled vibration damping device which is capable of effectively suppressing the cavitation, which may otherwise cause abnormal noises, without impairing the vibration damping effect.